IEEE - Aerospace and Electronic Systems - November 2019 - 9

LINK BUDGET ELEMENTS
Signaling: Optical Modulation and Coding
For deep-space communication, IM at the transmitter and
DD at the receiver are assumed, whereby the information is
signaled through PPM. That is, from a physical layer point
of view in each time slot the laser is either "ON" or "OFF".
In PPM, log2M bits are modulated by transmitting a
single pulse in one of the M possible time slots of a symbol, as shown in Figure 2 [11].
Ts ðsÞ is the symbol duration, Tslot (s) is the slot duration, and TLR ðsÞ is the laser recharge time duration or

Table 1.

State-of-the Art and 2023 Extrapolation Thereof for
RF Deep-Space Missions [3]

also referred to as guard time. These durations are related
as Ts ¼ M Á Tslot þ TLR . According to [4], the guard time
is defined as TLR ¼ M Á Tslot =4 meaning that the symbol
duration is Ts ¼ 5 Á M Á Tslot =4.
The coding and synchronization method defined in
[4] for the reliable delivery of data is based on SCPPM
[12]. In [12], SCPPM is introduced for deep-space communications, while it is tested against other coding techniques, demonstrating that SCPPM approaches the
capacity bound of the deep-space optical channel by a
couple of dBs at most. As typically in a deep-space mission there are several phases covering a wide range of
link distances, the [4] has been equipped with a wide
range of parameters. This includes the allowed values of
PPM order (4, 8, 16, 32, 64, 128, 256), of the code rate
(1/3, 1=2 , 2/3), of repeating the symbol to trade power
efficiency with data rate (1, 2, 3, 4, 8, 16, 32) and of the
slot widths (0.125 ns, 0.25 ns 0.5 ns, 1 ns, 2 ns, 4 ns, 8
ns, and 512 ns).

Mars
mission
(MEX XBand
35-m
Antenna)

Mars
mission
(2023
Frame,
KaBand)
35-m
Antenna

Jupiter
mission
(2017
Frame,
JUICE
X/KaBand)
35-m
Antenna

Jupiter
mission
(2023
Frame,
JUICE
X/KaBand)
35-m
Antenna

Data rate
at far
range

0.033
Mbps

2.65
Mbps

0.025
Mbps

0.370
Mbps

Pr;ap ¼ Pt Á Gt Á Gr Á Lfs Á La Á Lc Á Ls Á Lpt Á ht Á hr

Space
terminal
power

140 W

160 W

175 W

175 W

Space
terminal
massreliability

37 kg

45 kg

78 kg

45 kg

where Pt (Watt) is the transmitted power, Gt and Gr are
the transmitter and ground receiver aperture gains, respectively, Lfs is the free space loss factor, La is the atmospheric transmittance, Lc is the cirrus transmittance
factor, Ls is the scintillation loss factor Lpt is the pointing
loss factor, and ht and hr are the transmitter and receiver
efficiencies, respectively.

Ground
network

2 stations in two different hemisphere,
35 m antenna

NOVEMBER 2019

Received Power - Before the Photon Detector
In this section, the received power after the aperture
is given and the factors that impair the link are
exhibited.1
The received power after the receiver telescope and
before the photo detector Pr;ap (Watt) can be estimated
as
(1)

1
In this stage only the losses which are not dependent on the detector
are reported.

IEEE A&E SYSTEMS MAGAZINE

9
IEEE - Aerospace and Electronic Systems - November 2019

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